Radome

ABSTRACT

A radome for the transmission with minimal electrical distortion of circularly polarized electromagnetic energy and which allows circulation of warm air for de-icing. Two sheet-like elements, each having interior elongated, generally parallel, spaced ribs are joined together to form a laminate. The ribs of the first element are oriented generally perpendicular to the ribs of the second element to minimize elliptical distortion. Spaces within the first element and between the ribs thereof define warm air ducts. Attenuation of the electromagnetic energy is minimized by making the electrical thickness of each of the sheet-like elements approximately equal to one-half of the wavelength of the energy.

[111 3,87LQM [451 Mar. 11, 1975 United States Patent 1 Myers RADOME [75] Inventor: Hubert A. Myers, Los Angeles,

Calif.

[73] Assignee: Hitco, Irvine, Calif.

[22] Filed: Nov. 15, 1972 211 App]. No.: 306,884

[52] US. Cl. 343/872, 343/909 [51 1.11.0]. H0lq 1/42 [58] Field of Search 343/705, 708, 872, 909

[56] References Cited UNITED STATES PATENTS 2,755,216 7/1956 Lemons 343/872 2,840,811 6/1958 McMillan 343/872 3,432,859 3/1969 Jordan et al. 343/872 Primary ExaminerEli Lieberman Attorney, Agent, or FirmFraser and Bogucki [57] ABSTRACT A radome for the transmission with minimal electrical distortion of circularly polarized electromagnetic energy and which allows circulation of warm air for deicing. Two sheet-like elements, each having interior elongated, generally parallel, spaced ribs are joined together to form a laminate. The ribs of the first element are oriented generally perpendicular to the ribs of the second element to minimize elliptical distortion. Spaces within the first element and between the ribs thereof define warm air ducts. Attenuation of the electromagnetic energy is minimized by making the electrical thickness of each of the sheet-like elements approximately equal to one-half of the wavelength of the energy.

10 Claims, 13 Drawing Figures PATENTED 3.871.001

sum 2 05 5 FlG.-5

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/ ORIENTATION PATENTENN 1 1 3.87 1.001

SHEET 5 BF 5 I |oe /I04 RECEIVER I00 98 I02 SIGNAL as I08 GENERATOR PLACE FLAT ELEMENT TRANSM" TANGENT To POLARIZED ENERGY cuREo ELEMENT THROUGH ELEMENTS N ORIENT FLAT ELEMENT TO ELIMINATE msToNnoN REC*0RD FlG.-12 FlG. -'l3 1 RADOME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to radomes of the type typically used on aircraft and in similar applications to house radar transmission equipment, and more particularly to radomes of the type which are comprised of integrally woven, resin impregnated, three-dimensional panels having internal ribs which extend between opposite face sheets and define warm air ducts therebetween for de-icing purposes.

2. History of the Prior Art Radomes are used in aircraft and for other applications where it is desirable to physically protect transmission or receiving equipment from the environment while allowing the transmission of electromagnetic energy. Considerations involved in the design of a radome include the degree of attenuation of electromagnetic energy, physical strength and cost. Two additional considerations which are very important in particular radome applications are the extent to which the radome may distort the electromagnetic energy and the ability of the radome to prevent ice build-up.

In the past, radomes of high strength have been made by the construction of sandwich elements having a honeycomb core or interior. Another method of high strength construction has involved use of corrugated materials. Such materials have been used singly as well as in laminations wherein they are selectively oriented relative to one another to acheive high strength. Relatively strong radomes have also been built using materials which are filled with solid dielectric materials.

The minimization of attenuation is a basic goal in designing a radome. Some of the factors which affect attenuation are the material of construction, the thickness of the radome and ice buildup. Materials such as metal which would provide a radome of superior strength cannot be used because of their severe attenuation of electromagnetic energy. Consequently a dielectric material must generally be used although the strength of such materials is not as great as might be desired. The thickness of the radome affects attenuation as well as strength. The material used to fabricate the radome has a particular dielectric constant. For a given dielectric constant and a particular frequency of electromagnetic radiation, there is an optimal thickness in which there is maximum transmission of the electromagnetic signal and minimal internal reflection. A third cause of attenuation is build-up of ice on the radome which is a problem in many environments. The effect of the ice build-up is to add an additional dielectric interface between the air and the radome material, thereby increasing signal attenuation. If the radome is thin enough, heating of the interior surface is effective to de-ice the radome. However, strength requirements typically dictate against use of materials thin enough to be de-iced in this manner. Radomes which are thick enough to meet structural requirements are often provided with internal chambers or ducts through which warm air may be pumped to perform de-icing. However the additional material interfaces introduced by the presence of the chambers further add to attenuation problems.

A further problem of substantial importance in the design of radomes is distortion of the electromagnetic energy as it passes through the radome. Distortion is frequently caused by the material interfaces in radomes of the type having interior warm air ducts. The interior ribs forming such constructions commonly attenuate electric field vectors which are other than perpendicular thereto. Accordingly distortion and attenuation can be greatly minimized where linear polarization of the electromagnetic wave energy is present simply by orienting the internal ribs so that they are perpendicular to the direction of polarization. However, where the electromagnetic wave energy is circularly polarized, such compensation is not possible.

In the past, radomes have eliminated distortion and attenuation by' using sets of perpendicularly disposed metal plates defining square apertures. Each set of plates distorts the component of the circularly polarized wave in a direction parallel to the distorting plates. The distortion of the perpendicular components is about equal, resulting in an attenuated though undistorted wave. The squares between the plates are commonly filled with a dielectric material. Radomes of this type may be unduly complex and expensive, and are seldom equipped to perform reliable and effective deicing.

It is therefore an object of this invention to provide an improved radome.

It is another object of this invention to provide a radome which is relatively strong, inexpensive and easily de-iced, and yet which minimizes distortion and attenuation of electromagnetic wave energy.

It is an additional object of this invention to provide a radome which greatly minimizes the distortion and attenuation of circularly polarized electromagnetic energy.

BRIEF SUMMARY OF THE INVENTION Radomes in accordance with the invention comprise a laminate ofgenerally sheet-like elements oriented rel ative to one another so as to minimize distortion of electromagnetic energy passing therethrough. Each sheet-like element, which has a plurality of internal, generally parallel, spaced apart ribs defining therebetween warm air ducts for de-icing purposes, distorts electromagnetic energy in predictable fashion, which information is advantageously utilized in the minimization of distortion using a laminate of two such elements. Where the electromagnetic energy is circularly polarized the material interface introduced by the internal ribs and spaces therebetween in one of the sheetlike elements produces distortion which results in an elliptical pattern having a predictable orientation rela tive to the direction of the internal ribs. In accordance with the invention the internal ribs of the second sheetlike element are selectively oriented relative to the ribs of the first element to produce predictable distortion and a resulting elliptical pattern of the electromagnetic energy which tends to compensate for or balance out the distortion introduced by the first sheet-like element. In most instances the ribs of the second element are preferably oriented at approximate right angles with respect to the ribs of the first element. The practical result is a radome which is relatively strong, simple in construction, inexpensive and easy to de-ice, and yet which produces little or no distortion in circularly polarized electromagnetic energy passing therethrough.

In accordance with the invention, attenuation of electromagnetic energy passing through the radome is generally minimized by fabricating each of the sheetlike elements so as to have an equivalent electrical thickness approximately equal to one-half of a wavelength of the electromagnetic energy at the operating frequency. Sheet-like elements which are so proportioned produce practically no reflection of the energy as it passes therethrough, resulting in little or no attenuation.

Methods of making a radome in accordance with the invention involve placing the two sheet-like elements together and varying their relative orientation until distortion of electromagnetic energy passed therethrough is minimized, whereupon the elements are bonded together in such position. The sheet-like element on the outside of the radome may be fabricated of a woven cloth which is resin impregnated and cured, and preferably has internal spaces which are left open to serve as warm air ducts by inserting mandrels therein during the impregnation process and removing the mandrels after curing. The sheet-like element on the inside of the radome may be fabricated in similar fashion, except that the open spaces therein are typically filled with a foamlike material for added strength and insulation.

DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings, in which:

FIG. I is an exploded, partly broken away perspective view of a portion of a laminate for use in the construction of radomes in accordance with the invention;

FIG. 2 is a section view of the laminate of FIG. 1 taken along the line 2-2 of FIG. I;

FIG. 3 is a section view of the laminate of FIG. 1 taken along the line 3-3 of FIG. 1;

FIG. 4 is a schematic representation of the behavior of a linearly polarized electromagnetic wave as it passes through a portion of the laminate of FIG. I while oriented in a first direction;

FIG. 5 is a schematic representation of the behavior ofa linearly polarized electromagnetic wave as it passes through a portion of the laminate of FIG. I while oriented in a second direction;

FIG. 6 is a schematic representation of the behavior of a circularly polarized electromagnetic wave as it passes through a portion of the laminate of FIG. I;

FIG. 7 is a schematic representation of the behavior of a circularly polarized electromagnetic wave as it passes through the laminate of FIG. 1;

FIG. 8 is a sectional view of an alternative form of laminate in accordance with the invention;

FIG. 9 is a sectional view of a radome and a portion of radar transmission apparatus mounted therein illustrating the manner in which the angle of incidence of the electromagnetic wave energy with respect to the radome may vary;

FIG. 10 is a block diagram comprising the various steps in one preferred method of making a radome in accordance with the invention;

FIG. 11 is a schematic representation ofa method of obtaining an optimal orientation of rib structures;

FIG. 12 is a perspective view with portions removed of a radome having an element formed from wedges of fluted core material; and

FIG. 13 is a block diagram of a method of obtaining an optimal orientation of rib structures.

DETAILED DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 illustrate a preferred form of a laminate I0 in accordance with the invention. The laminate I0 comprises first and second generally planar, sheetlike elements 12 and 14 which are shown separted in FIG. I to illustrate the details thereof but which are bonded together to form the finished laminate III as shown in FIGS. 2 and 3. Sheet-like element I2 and sheet-like element 14 are preferably integrally woven glass cloth which has been resin impregnated, though it should be noted that other materials may be used. The glass cloth in suitable since it is a relatively low loss dielectric. Also, it is flexible and easy to mold. This is an important consideration since most radomes must be curbed or hyperbolic in shape. Resin is used both to provide structural rigidity and render the glass cloth impermeable to the elements. The resin is also employed to bond the elements I2 and 14 together, as described hereinafter.

The element I2 comprises a pair of generally planar, spaced-apart face sheets I6 and I3, which are joined together by a plurality of intermediate, mutually paral lel, spaced-apart ribs 20.

Similarly the element .14 comprises a pair of generally planar, spaced-apart face sheets 22 and 24 which are joined together by a plurality of intermediate, generally parallel, spaced-apart ribs 26.

The ribs 20 of the element 12 combine with the face sheets 16 and 18 thereof to form a plurality of apertures 28 therein. In similar fashion a plurality of aper tures 30 are formed within the element 14. The various apertures 28 and 30 can function as warm air ducts so as to de-ice the laminate II), although in actual practice only the apertures 30 need be so used where the ele ment I4 forms the outer skin of the radome. An exhaust manifold (not shown) may be coupled to the apertures 30 to circulate warm air therethrough in conventional fashion.

The relatively thick woven structure comprising the elements I2 and I4 provides for a relatively strong and rigid radome while at the same time minimizing the bulk, weight and expense of such structure. The apertures 28 and 30 are ideally suited to de-icing of the radome. However these advantages are achieved at the expense of distortion and attenuation of electromag netic wave energy which can occur due to the apertures 28 and 30 and particularly to the ribs 20 and 26 forming the apertures. While air within the apertures 28 and 30 has a dielectric constant of I, the material comprising the woven structures 12 and I4 typically has a dielectric constant on the order of 4.5. The resulting interfaces of the ribs 20 and 26 between the apertures 28, 30 and the various face sheets I6, IS, 22 and 24 can cause highly undesirable distortion, depending on the manner in which the electromagnetic wave energy is polarized and the manner in which it passes through the elements. FIG. 4 depicts a vertically polarized electro magnetic wave 32 as it passes through the element 12. An arrow 34 represents the direction of propagation of the electromagnetic wave 32 and a plurality of arrows 36 represent the electric field or E vector of the electromagnetic wave 32 as the wave 32 propagates into the element I2 along a vertical plane of polarization. A plurality of arrows 38 represent the E vector of the electromagnetic wave 32 after it has passed through the element I2. The various ribs 20 and apertures 28 of the element 12 are generally parallel to the direction or plane of polarization of the wave 32. As seen in FIG. 4 the E vectors 38 still lie within a vertical plane of polarization after passage through the element 12. However the normal orientation of the ribs 20 and apertures 28 relative to the plane of polarization results in severe attenuation of the wave 32.

FIG. 5 depicts a situation in which the element 12 is' oriented in the same way as in FIG. 4 but with a horizontally polarized wave 40 passing therethrough. An arrow 42 represents the direction of propagation and arrows 44 represent the E bector defining a horizontal plane of polarization. As seen in FIG. 5 the E vector of the wave 40 after the wave has passed through the element 12 and as represented by a plurality of arrows 46, continues to lie in the horizontal plane of polarization and is approximately equal in size to the vectors 44 showing that almost no attenuation has taken place.

A circularly polarized electromagnetic wave 48, in FIG. 6, is passed through the sheet-like element 12. The direction of propagation is represented by an arrow 50, while the rotating E vector as the wave 48 propagates into the element 12 is represented by a plurality of arrows 52. As represented by a circle 54 the E vector of the wave 48 defines a generally circular pattern prior to passage through the element 12. However upon passage through the element 12 the wave 48 undergoes varying degrees of attenuation depending upon the orientation of the E vector and as shown by a plurality of arrows 56. Thus E vectors pointing in a substantially horizontal direction receive little or no attenuation, while E vectors pointing in a substantially vertical direction undergo extensive attenuation. The resulting pattern is generally elliptical in shape as represented by an ellipse 58.

Many radar systems can tolerate some attenuation. Where attenuation poses serious problems, equipment can always be provided to amplify the attenuated sig nal. However, distortion of the type illustrated in FIG. 6 is highly undesirable and should be compensated for. One form of compensation is to resort to a material for the radome which does not have elongated apertures therein such as the apertures 28 and which therefore causes little or no distortion. However, as previously noted such materials can be expensive, have undue bulk, often lack adequate strength and, above all, are difficult to de-ice.

In accordance with the invention laminates of the sheet-like elements such as the laminate 10 of FIGS. l-3 are advantageously utilized with circularly polarized electromagnetic waves without unwanted distortion and with little or no attenuation where properly designed. FIG. 7 shows the same wave 48 of FIG. 6 as it propagates through the sheet-like element 12 to become elliptically distorted as represented by a dashed ellipse 60 on the other side of the element 12. However, the wave as so distorted is in effect re-distorted back into its original circular configuration by the second sheet-like element 14 which has the ribs 26 and the apertures 30 thereof oriented generally perpendicular to the ribs 20 and apertures 28 of the first sheet-like element 12. A plurality of arrows 62 represent the E vector of electromagnetic wave 48 after it has passed through the laminate 10. Here in E vector 62 has been attenuated. However, unlike the arrows 56 representing the E vector of the electromagnetic wave 48 after passing through the single sheet-like element 12 of FIG.

6, the E vector of FIG. 7 has been attenuated in all directions. Thus, the distortion created by the sheet-like element 12 is cancelled by distortion introduced by the sheet-like element 14, resulting in an undistorted though somewhat attenuated circularly polarized electromagnetic wave as represented by a circle 64.

In accordance with the invention the attenuation of electromagnetic wave energy produced by the laminate l0, and as represented by the relative sizes of circles 54 and 64 in FIG. 7, is minimized by making the thickness of each of the elements 12 and 14 such that the reflections from the face sheets are cancelled at the input surface 17 of the electromagnetic wave. This thickness is a function of the dielectric constant and physical thickness of the individual face sheets and is approximately an equivalent electrical half wavelength in free space. When this condition previals the circle 64 in FIG. 7 is approximately equal in size to the circle 54. Attenuation is minimized at an element thickness of an electrical equivalent half wavelength in free space since practically all reflections caused by the interfaces of the radome material with air or other dielectrics are matched. On the other hand, such interfaces produce maximum reflection and hence substantial attenuation when the face sheets are spaced odd increments of equivalent of )t/4 in free space, greater or less than the spacing of the face sheets 16, 18 for minimal attenuation.

In designing a laminate 10 for potential radome installation, the operating frequency is taken into account. Thus where operation is to be in the X band of microwave frequencies, )t is approximately 1.25 inches in free space. However the wave energy is slowed down somewhat by each element 12 and 14, along with the interface reflections to produce an electrical wavelength which is shorter than the actual free space wavelength. At the X band the electrical wavelength has been found to be on the order of 0.646 inch. Thus, each element 12 and 14 is desirably made approximately 0.323 inch thick in such a situation. This of course is dependent on the dielectrical constant and the mechanical thickness of the individual face sheets.

The elements 12 and 14 shown and described thus far comprise A type panels or sandwiches in which the dielectric constant of the woven structure is higher than that of the apertures (air). The principles of the invention are equally applicable to B type panels or sandwiches in which the dielectric constant of the woven structure is lower than that of the apertures, usually due to a material of relatively high dielectric constant being placed within the apertures. However, B type panels have the disadvantage that the apertures are not available for use as warm air ducts in de-icing.

FIG. 8 depicts a modified A panel in the form of an element 66 in accordance with the invention. The element 66 is similar to the laminate 10 in that it has outer face sheets 68 and 70. However the dual inner face sheets of the laminate 10 are replaced by a single face sheet 72 in the element 66, and the face sheet is made considerably thinner than the face sheets 68 and 72. This allows warm air which may circulate through plural apertures 74 to be in close proximity to an exterior surface 76 of the element 66.

Since the technique of weaving intergrally woven fluted core glass cloth is well known in the art and allows for any combination of face sheet thickness, rib thickness, face sheet spacing and rib spacing, a radome can be designed and constructed for any frequency within practical limits. Further, since it is possible to use very thin face sheets and thick ribs, it is possible to design a high transmission efficiency into the radome and still maintain good ellipticity characteristics.

The various apertures 28 and 30 within the elements 12 and 14 are illustrated as being generally square in crosssection. In actual practice the apertures may be rectangular in cross-section and may vary in relative dimensions, although it is preferred that the length of each aperture of rectangular cross-section not exceed more than twice the width of such aperture. The apertures may also assume other appropriate shapes and may, for example, be triangular in cross-section where dictated by manufacturing or other factors.

If the face sheet 22 of the element 14 is assumed to define the outer surface of the radome, then the apertures 30 are used as warm air ducts to de-ice the radome. At the same time, the apertures 28 within the element 12 which are not needed for de-icing are typically filled with a light weight foam to enhance the strength and insulative properties of the radome.

In practice, radomes in accordance with the invention are generally hyperbolic in shape as seen in FIG. 9. As seen in FIG. 9 the elements 12 and 14 comprising the laminate 10 must assume such a hyperbolic shape so as to surround and enclose radar transmission apparatus 78 including a movable dish 80. When the dish 80 is in a central position as shown, it transmits and receives electromagnetic wave energy along an axis 82 which is generally normal to the end portion 84 of the hyperbolic radome. Accordingly, the ribs of the elements l2 and 14 are preferably oriented generally at right angles to one another in the region of the end portion 84 to minimize distortion. However since the dish 80 is pivotable, the transmission axis thereof may have an angle of incidence with respect to the radome surface of other than 90. For example when the dish 80 assumes the position shown by the dotted outline 86 in FIG. 9, the dish 80 has a transmission axis 88 which intersects the radome at an angle considerably different from 90. It has been found that in regions of the radome such as this the elements 12 and 14 must usually be r eatedats neaisle.gt r than 9 mtia ln zq distortion. Thus the relative angles between the ribs of the elements 12 and 14 may vary between 65 and 115 for different locations throughout the radome. During the method of fabrication described hereafter the element 12 may be laid loosely within the formed element 14 and the orientation thereof relative to the element 14 varied at different surface locations until distortion is minimized as determined by a signal strength meter.

Referring now to FIG. 10, one method for making a radome in accordance with the invention is disclosed. Initially, the inner and outer elements 12 and 14 are woven using appropriate conventional techniques. Thereafter mandrels are inserted within the apertures of the outer element 14 which is then laid-up by impregnating it with resin and placing it within a mold having the desired shape of the radome. Following curing of the outer element 14, the mandrels are removed so that the apertures may be used as warm air ducts. The inner element 12 is then prepared by making foam fillers of appropriate size, inserting the foam fillers in the apertures of the element 12 with the help of mandrels where necessary, and resin impregnating the element 12. The element 12 as so prepared is laid-up on the inside of the formed outer element 14, and has the ribs thereof very carefully oriented relative to the ribs of the formed outer element 14 to minimize distortion in accordance with the invention. With the element 12 so located, the combined laminate is cured so as to harden the resin and rigidify the element 12 while at the same time bonding the element 12 to the element 14.

FIGS. 11, 12 and 13 depict a method for properly orienting the ribs of the sheet-like elements of a generally curved radome. Initially, the element 12 is resin impregnated and cured. The element 12 forms a base for the adjacent coextensive element 14 (not shown). An antenna structure is located within the element 12. A corresponding antenna structure 92 is placed external to the element 12 so as to receive electromagnetic wave energy transmitted by the structure 90. A preferably flat, planar, sheet-like element 94 is similar in construction to the elements 12 and 14 and comprises a pair of generally planar, spaced apart face sheets 96 and 98, which are joined together by a plurality of in termediate, mutually parallel spaced-apart ribs 100. The element 94 is laid upon the radome surface tangent to sheet-like element 12 at an area of incidence of electromagnetic waves propagating between the antenna structures 90 and 92. Circularly polarized energy from a signal generator 102, transmitted through the elements 12 and 94 is received having an eccentricity depending upon the orientation of the ribs 100 of element 94 with respect to ribs 20 of the element 12. Alternately, linearly polarized energy from the signal generator 102 is transmitted through the elements 12 and 94 while antenna structure 90 is slowly revolvedv The dis tortion of the polarized energy may be conveniently viewed on an oscilloscope 104 coupled to the antenna structure 92 through a receiver 106. Alternately, the directional attenuation or distortion may be observed on a voltmeter. The element 94 is then rotated about a line 108 normal to the plane of tangency between the elements 12 and 94 until elliptical distortion is eliminated. The proper orientation of the ribs for sheet-like element 14 (not shown) is thus obtained for the general area of tangency. This orientation information is then recorded. The sheet-like elements are somewhat translucent when made of fiberglass materials, and orienta tion information is easily marked on the interior of the sheet like element 12. After a number of similar opera tions of orienting and recording are performed at vari ous regions of the element 12, the rib orientation for the entire radome is thereby obtained.

The sheet-like elements are generally formed from a flexible fluted core material with substantially parallel orientation of the ribs. It is most practical to cut the fluted core material into, for example, pie-shaped wedges 110, as depicted in FIG. 12, in order to fit the fluted core material to approximate the obtained rib orientation. Thus the ribs of adjacent wedges are fitted to element 12 in accordance with the recorded orientation. Also, to enable mandrels (not shown) to be used to set the shape of the outer element, channels 112 between the ribs of wedges 110 are aligned.

It should be recognized that either the inner element 12 or the outer element 14 may be laid up first as a base element. Thus, the wedges 110 may be set up and positioned either on the interior or the exterior of the base element. Also, it should be understood that the angular orientations of antenna structure 90 and 92 are not critical. Further, transmission of polarized energy may be from either external antenna structure 92 to the internal antenna structure 90 or from the internal antenna structure 90 to the external antenna structure 92.

It should be recognized that other modes of this invention will become apparent to one skilled in the art. It is therefore contemplated that the claims cover all modifications within the spirit of this invention.

What is claimed is:

l. A radome comprising first and second generally sheet-like elements joined together to form a laminate, each of the sheet-like elements having opposite broad surfaces and a plurality of elongated spaces disposed within the sheet-like element intermediate the opposite broad surfaces and spaced-apart across the sheet-like element to define a plurality of elongated ribs therebetween, the elongated spaces and the ribs formed therebetween extending generally in a common direction, each of the sheet-like elements distorting electromagnetic wave energy which passes therethrough in predictable fashion, the ribs of the first sheet-like element being oriented relative to the ribs of the second sheetlike element so that distortion of electromagnetic wave energy which has passed through the first sheet-like element is substantially balanced by distortion of the electromagnetic wave energy after it has passed through the second sheet-like element.

2. The invention defined in claim 1, wherein the electromagnetic wave energy is circularly polarized, and each of the first and second sheet-like elements distorts the electromagnetic wave energy into a generally elliptical pattern, the orientation of which is determined by the direction of the included ribs.

3. A radome comprising first and second generally sheet-like elements joined together to form a laminate, each of the sheet-like elements having a plurality of elongated, generally parallel, spaced-apart internal ribs therein, each of the first and second sheet-like elements including a pair of nominally parallel face sheets disposed on opposite sides of and sandwiching the ribs therebetween, each of the sheet-like elements distorting electromagnetic wave energy which passes therethrough in predictable fashion, the ribs of the first sheet-like element being oriented relative to the ribs of the second sheet-like element so that the distortion of electromagnetic wave energy which has passed through the first sheet-like element is substantially balanced by distortion of the electromagnetic wave energy after it has passed through the second sheet-like element.

4. A radome comprising first and second generally sheet-like elements joined together to form a laminate,

10 each of the sheet-like elements having a plurality of elongated, generally parallel, spaced-apart internal ribs therein, each of the first and second sheet-like elements having an electrical thickness equal to approximately one-half of a wavelength of the electromagnetic wave energy, each of the sheet-like elements distorting electromagnetic wave energy which passes therethrough in predictable fashion, the ribs of the first sheet-like element being oriented relative to the ribs of the second sheet-like element so that distortion of electromagnetic wave energy which has passed through the first sheet like element is substantially balanced by distortion of the electromagnetic wave energy after it has passed through the second sheet-like element.

5. A radome comprising first and second generally parallel sandwiches, each of the sandwiches comprising a pair of parallel, spaced apart face sheets with a plurality of elongated, generally parallel, spaced-apart ribs extending therebetween, one of the face sheets of the first sandwich being mounted on and generally coextensive with a face sheet of the second sandwich, and the ribs of the first sandwich being oriented relative to the ribs of the second sandwich so as to minimize distortion of electromagnetic wave energy passing through the first and second sandwiches.

6. The invention defined in claim 5, wherein the thickness of each sandwich in a direction normal to the direction of the plane thereof is equal to approximately one-half of an electrical wavelength of the electromagnetic wave energy.

7. The invention defined in claim 5, wherein spaces formed between the face sheets and intermediate the ribs of the first sandwich define warm air ducts for deicing of the radome, and wherein spaces formed between the face sheets and intermediate the ribs of the second sandwich are filled with foam.

8. The invention defined in claim 5, wherein each of the face sheets and the ribs of each sandwich together comprise interwoven, resin impregnated cloth.

9. The invention defined in claim 5, wherein the face sheets which are mounted on and coextensive with each other have a combined thickness less than twice the thickness of each of the opposite face sheets.

10. The invention as defined in claim 5, wherein the face sheets which are mounted on and coextensive with each other comprise a single face sheet having a thickness approximately equal to thickness of one of the opposite face sheets and substantially greater than the thickness of the other one of the opposite face sheets. l= l UNITED STATES PATENT OFFICE 1 CERTIFICATE OF CORRECTION PATENT NO. 1 3,871,001 DATED March 11, 1975 INVENTOR(S) Hubert A. Myers it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 31, for "acheive" read --achieve--. Column 4, line 6, for "separted" read --separated--; line 13, after "cloth" strike "in" and insert --is--; line 16, for "curbed" read --c'urved--. Column 5 line 64, after "Here" strike "in" and insert --the-- Column '6, line 64, for "intergrally" read --integrally--. Column 7, line 8, for

"crosssection" read --cross-sectim-- Column 10, line 47, after "to" insert --the--.

Signed and sealed this 29th day of April 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Comissioner of Patents Attesting Officer and Trademarks 

1. A radome comprising first and second generally sheet-like elements joined together to form a laminate, each of the sheet-like elements having opposite broad surfaces and a plurality of elongated spaces disposed within the sheet-like element intermediate the opposite broad surfaces and spaced-apart across the sheet-like element to define a plurality of elongated ribs therebetween, the elongated spaces and the ribs formed therebetween extending generally in a common direction, each of the sheet-like elements distorting electromagnetic wave energy which passes therethrough in predictable fashion, the ribs of the first sheet-like element being oriented relative to the ribs of the second sheet-like element so that distortion of electromagnetic wave energy which has passed through the first sheet-like element is substantially balanced by distortion of the electromagnetic wave energy after it has passed through the second sheet-like element.
 1. A radome comprising first and second generally sheet-like elements joined together to form a laminate, each of the sheetlike elements having opposite broad surfaces and a plurality of elongated spaces disposed within the sheet-like element intermediate the opposite broad surfaces and spaced-apart across the sheet-like element to define a plurality of elongated ribs therebetween, the elongated spaces and the ribs formed therebetween extending generally in a common direction, each of the sheet-like elements distorting electromagnetic wave energy which passes therethrough in predictable fashion, the ribs of the first sheet-like element being oriented relative to the ribs of the second sheet-like element so that distortion of electromagnetic wave energy which has passed through the first sheet-like element is substantially balanced by distortion of the electromagnetic wave energy after it has passed through the second sheet-like element.
 2. The invention defined in claim 1, wherein the electromagnetic wave energy is circularly polarized, and each of the first and second sheet-like elements distorts the electromagnetic wave energy into a generally elliptical pattern, the orientation of which is determined by the direction of the included ribs.
 3. A radome comprising first and second generally sheet-like elements joined together to form a laminate, each of the sheet-like elements having a plurality of elongated, generally parallel, spaced-apart internal ribs therein, each of the first and second sheet-like elements including a pair of nominally parallel face sheets disposed on opposite sides of and sandwiching the ribs therebetween, each of the sheet-like elements distorting electromagnetic wave energy which passes therethrough in predictable fashion, the ribs of the first sheet-like element being oriented relative to the ribs of the second sheet-like element so that the distortion of electromagnetic wave energy which has passed through the first sheet-like element is substantially balanced by distortion of the electromagnetic wave energy after it has passed through the second sheet-like element.
 4. A radome comprising first and second generally sheet-like elements joined together to form a laminate, each of the sheet-like elements having a plurality of elongated, generally parallel, spaced-apart internal ribs therein, each of the first and second sheet-like elements having an electrical thickness equal to approximately one-half of a wavelength of the electromagnetic wave energy, Each of the sheet-like elements distorting electromagnetic wave energy which passes therethrough in predictable fashion, the ribs of the first sheet-like element being oriented relative to the ribs of the second sheet-like element so that distortion of electromagnetic wave energy which has passed through the first sheet-like element is substantially balanced by distortion of the electromagnetic wave energy after it has passed through the second sheet-like element.
 5. A radome comprising first and second generally parallel sandwiches, each of the sandwiches comprising a pair of parallel, spaced apart face sheets with a plurality of elongated, generally parallel, spaced-apart ribs extending therebetween, one of the face sheets of the first sandwich being mounted on and generally coextensive with a face sheet of the second sandwich, and the ribs of the first sandwich being oriented relative to the ribs of the second sandwich so as to minimize distortion of electromagnetic wave energy passing through the first and second sandwiches.
 6. The invention defined in claim 5, wherein the thickness of each sandwich in a direction normal to the direction of the plane thereof is equal to approximately one-half of an electrical wavelength of the electromagnetic wave energy.
 7. The invention defined in claim 5, wherein spaces formed between the face sheets and intermediate the ribs of the first sandwich define warm air ducts for de-icing of the radome, and wherein spaces formed between the face sheets and intermediate the ribs of the second sandwich are filled with foam.
 8. The invention defined in claim 5, wherein each of the face sheets and the ribs of each sandwich together comprise interwoven, resin impregnated cloth.
 9. The invention defined in claim 5, wherein the face sheets which are mounted on and coextensive with each other have a combined thickness less than twice the thickness of each of the opposite face sheets. 